The present invention relates to a wash-primer base paint composition, and more particularly to such a composition, to be mixed with a catalyst composition, to produce a wash-primer which may be applied to the surface of metals, such as galvanized steel, stainless steel and cast iron, aluminum and aluminum alloys, to inhibit the spread of corrosion and to act as a tie-coat.
Wash-primers representing traditional formulations are presently used as surface conditioners to inhibit the spread of corrosion and as tie-coat, mainly for galvanized steel or light metals. The range of substrates which can be coated with conventional wash-primers is much broader. Wash-primers are suitable as pre-treatment coat for substrates such as steel, stainless steel and cast iron, an aluminum and aluminum alloys. Such primers are usually marketed as a two-can (two component) material consisting of a base paint, (pigmented part A) and an acidic or catalyst component (clear part B) which are mixed prior to use, in a weight ratio ranging between 85:15 and 80:20. A standard formulation for such a wash-primer is as follows:
______________________________________ Parts by Weight ______________________________________ Part A Polyvinyl butyral resin 30.50 Zinc tetraoxy chromate 29.40 Magnesium silicate 4.40 Lamp black 0.33 n-butanol 68.00 Ethanol (denatured) 207.00 Total: 339.63 Part B Phosphoric acid (85%) 15.3 Water (max.) 13.6 Ethanol (denatured) 57.1 Total: 86.0 ______________________________________
The mixing ratio is 80:20 by weight.
The rust inhibitive and metal substrate prepriming properties of such chromate pigmented wash-primers are based on the following functions of the components:
chromate ions formed in the wash-primer coating, on contact with moisture, provide corrosion inhibition of ferrous substrates;
zinc ions and phosphoric acid tend to create zinc phosphate conversion coating on ferrous substrates and etching effect on many other metals;
the in-situ cross-linked film of polyvinyl butyral resin has excellent adhesion to a variety of metallic substrates, thus providing a strong tie-coat for subsequent organic coatings; and
the oxidative properties of chromate ions tend to build up oxide layers on aluminum substrates thus improving their resistance to corrosive environments.
The above functions are achieved in a sequence which take place after the chromate pigmented base part of the wash-primer and the catalyst, which contains phosphoric acid, are mixed together and applied over a metallic substrate:
phosphoric acid converts part of the zinc tetraoxy chromate pigment to chromic acid, zinc phosphate and other chromates of lower basicity;
primary and/or secondary alcohols, present in the mixture, are oxidized by chromic acid to their respective aldehydes and ketones which take part in crosslinking reactions of polyvinyl butyral resin;
phosphoric acid and/or its zinc salts affect metallic substrates either by depositing respective phosphate films or by etching the surface;
chromic phosphate which is formed in the coating material during these processes forms a chelated complex with the polyvinyl butyral resin matrix creating a film that is strongly bonded to the deposited phosphate conversion coating by primary and/or secondary bonds; and
free chromates and phosphates remaining in the film serve as a source of inhibitive ions to protect the system against corrosion.
It is well known that the chromates which form an integral part of such conventional wash-primer compositions, are carcinogens and as such pose a potential hazard or danger to the health of persons who are applying them or to persons who are working with metals that have been coated with them.
It is an object of the present invention to provide a wash-primer composition to be used for coating various metallic substrates, which does not employ chromate pigments and yet which provides the same of equivalent performance to conventional, chromate-containing wash-primer compositions.